As a next-generation system of the W-CDMA (Wideband Code Division Multiple Access) and the HSDPA (High Speed Downlink Packet Access) which are collectively known as a UMTS (Universal Mobile Telecommunications System), an LTE system has been studied by 3GPP (3rd Generation Partnership Project) which is a standards body of UMTS. In the LTE system as a radio access system, an OFDM (Orthogonal Frequency Division Multiplexing) system and an SC-FDMA (Single-Carrier Frequency Division Multiple Access) system have been studied to be applied to the downlink communication system and the uplink communication system, respectively (see, for example, Non Patent Document 1).
In a mobile communication system, an information item indicating the location of user equipment (hereinafter, the user equipment may be referred to as a user equipment terminal) waiting for a call (in idle mode) is associated with the user equipment terminal, and the associated information is stored by using a tracking area as a unit. The tracking area corresponds to a Location Area (LA) in the UMTS or a Routing Area (RA).
In each cell, an identifier indicating the tracking area to which the cell belongs is reported. The tracking area to which a user equipment terminal in idle mode belongs is registered by a location registration. A user equipment terminal stores the identifier indicating the tracking area registered by the location registration. Further, upon being moved, the user equipment terminal receives the reported identifier of the tracking area and determines whether the tracking area is changed. For example, when the user equipment terminal is moved from a cell (first cell) to an adjacent cell (second cell) and then determines that a received identifier indicating the tracking area reported in the second cell is different from the identifier indicating the tracking area that is currently registered, the user equipment terminal accesses by using an uplink and performs the updates the location registration. The update of the location registration is called a Tracking Area Update (TAU). To perform the TAU, it is necessary for the user equipment terminal to use an uplink. Therefore, if the frequency (number) of performing the tracking area update (TAU) increases, the energy of the battery of the user equipment terminal may be wasted even when the user equipment terminal is in idle mode. Because of the feature, from the viewpoint of reducing the burden of the tracking area update (TAU), it may be preferable to increase the size of the tracking area.
When there is a call for a user equipment terminal, the mobile communication network pages all the cells of the tracking area currently registered as the tracking area of the user equipment terminal. Upon being paged, the user equipment terminal accesses (i.e., transmits a service request to) the cell where the user equipment terminal is located (hereinafter, the cell may be referred to as a serving cell) so that the call is established. However, the paging is performed even in the cells where the user equipment terminal is not located. Therefore, when the number of cells where the paging is performed is too many, many downlink radio resources may be of no use (wasted). From the viewpoint of reducing the overhead of the paging, it may be preferable to reduce the size of the tracking area. As described above, there is a trade-off relationship between the burden of performing the tracking area update (TAU) and the overhead of the paging. Therefore, it may be necessary to adequately determine (design) the tracking area (more specifically a correspondence relationship (combination) between the tracking area and the cells belonging to the tracking area).    Non Patent Document 1: 3GPP TR 25.814(V7.0.0) “Physical Layer Aspects for Evolved UTRA,” June 2006    Non Patent Document 2: 3GPP R2-061929 June 2006